Electrochemical battery cells that use a fluid, such as oxygen or other gas(es), from outside the cell as an active material to produce electrical energy, such as air-depolarized, air-assisted and fuel cell battery cells, can be used to power a variety of electronic devices. For example, air containing oxygen enters into an air-depolarized or air-assisted cell, where it can be used as, or can recharge, the positive electrode active material. The oxygen reduction electrode promotes the reaction of the oxygen with the cell electrolyte and, ultimately the oxidation of the negative electrode active material with the oxygen. The material in the oxygen reduction electrode that promotes the reaction of oxygen with the electrolyte is often referred to as a catalyst. However, some materials used in oxygen reduction electrodes are not true catalysts because they can be at least partially reduced, particularly during periods of relatively high rate discharge.
One type of air-depolarized cell is a zinc/air cell. This type of cell uses zinc as the negative electrode active material and has an aqueous alkaline (e.g., KOH) electrolyte. Manganese oxides that can be used in zinc/air cell air electrodes are capable of electrochemical reduction in concert with oxidation of the negative electrode active material, particularly when the rate of diffusion of oxygen into the air electrode is insufficient. These manganese oxides can then be reoxidized by the oxygen during periods of lower rate discharge or rest.
Air-assisted cells are hybrid cells that contain consumable positive and negative electrode active materials as well as an oxygen reduction electrode. The positive electrode can sustain a high discharge rate for a significant period of time, but through the oxygen reduction electrode, oxygen can partially recharge the positive electrode during periods of lower or no discharge, so oxygen can be used for a substantial portion of the total cell discharge capacity. This means the amount of positive electrode active material put into the cell can be reduced and the amount of negative electrode active material can be increased to increase the total cell capacity. Examples of air-assisted cells are disclosed in U.S. Pat. No. 6,383,674 and U.S. Pat. No. 5,079,106.
An advantage of air-depolarized, air-assisted and fuel cells is their high energy density, since at least a portion of the active material of at least one of the electrodes comes from or is regenerated by a fluid (e.g., a gas) from outside the cell.
A disadvantage of these cells is that the maximum discharge rates they are capable of can be limited by the rate at which oxygen can enter the oxygen reduction electrode. In the past, efforts have been made to increase the rate of oxygen entry into the oxygen reduction electrode and/or control the rate of entry of undesirable gases, such as carbon dioxide, that can cause wasteful reactions, as well as the rate of water entry or loss (depending on the relative water vapor partial pressures outside and inside the cell), that can fill void space in the cell intended to accommodate the increased volume of discharge reaction products or dry the cell out, respectively. Examples of these approaches can be found in U.S. Pat. Nos. 6,558,828; 6,492,046; 5,795,667; 5,733,676; U.S. Patent Publication No. 2002/0150814; and International Patent Publication No. WO 02/35641. However, changing the diffusion rate of one of these gases generally affects the others as well. Even when efforts have been made to balance the need for a high rate of oxygen diffusion and low rates of CO2 and water diffusion, there has been only limited success.
At higher discharge rates, it is more important to get sufficient oxygen into the oxygen reduction electrode, but during periods of lower discharge rates and periods of time when the cell is not in use, the importance of minimizing CO2 and water diffusion increases. To provide an increase in air flow into the cell only during periods of high rate discharge, fans have been used to force air into cells (e.g., U.S. Pat. No. 6,500,575), but fans and controls for them can add cost and complexity to manufacturing, and fans, even micro fans, can take up valuable volume within individual cells, multiple cell battery packs and devices.
Another approach that has been proposed is to use microvalves to control the amount of air entering the cells (e.g., U.S. Pat. No. 6,641,947 and U.S. Patent No. 6,773,842), but external means, such as fans and/or relatively complicated electronics can be required to operate the microvalves.
Yet another approach has been to use a water impermeable membrane between an oxygen reduction electrode and the outside environment having flaps that can open and close as a result of a differential in air pressure, e.g., resulting from a consumption of oxygen when the battery is discharging (e.g., U.S. Patent Publication No. 2003/0049508). However, the pressure differential may be small and can be affected by the atmospheric conditions outside the battery.
Additional approaches utilizing microvalves to control the amount of gas entering a cell are set forth in U.S. Pat. Nos. 5,304,431; 5,449,569; 5,541,016; and 5,837,394; incorporated herein by reference, wherein the microvalves are formed utilizing a silicon or semiconductor substrate and etching, deposition and micromachining processes.
Microactuators are further described in U.S. Pat. Nos. 4,969,938; 5,069,419; 5,271,597; and publications such as “Fluister: semiconductor microactuator” described in Instruments and Apparatus News (IAN), October 1993, p. 47, and Electronic Design, Nov. 1, 1993, p. 3.